1. Field of the Invention
The present invention relates to an image pickup apparatus of a field sequential record type for picking up a stereo image with right and left parallaxes.
2. Related Background Art
A stereo image pickup system will be described.
The stereo image pickup system described herein is of a field time-division type in which odd and even fields of an interlace video method such as NTSC are allocated to an image seen from a right eye (left eye) and an image seen from a left eye (right eye).
Images picked up and recorded by the field time-division method are reproduced on a CRT monitor or the like, and viewed by using a pair of liquid crystal shutter spectacles which open and close synchronously with image timings. In this manner, a stereo image of the picked-up image can be viewed.
FIG. 9 is a diagram showing the structure of a video camera to which the invention is applied. Referring to FIG. 9, reference numeral 500 represents a lens unit (optical system unit) for generating right and left parallaxes, this optical system being separated into right and left optical systems on the side of the objective lens. A liquid crystal shutter (for right eye) 502R and a liquid crystal shutter (for left eye) 502L disposed respectively in the right and left optical systems are operated so as to transmit a light flux (for right eye) 501R and a light flux (for left eye) 501L, respectively.
Transmitted light fluxes are converged into one light flux by a mirror (for right eye) 503R, a mirror (for left eye) 503L and a prism 504. The converged light flux passes through an iris 505 and through a lens exchange mount 802 via an optical system 506 constituted of lenses and the like, and is focussed on an image pickup plane of a CCD (image pickup element) 621 on the side of a camera unit 600. Only one of the right and left fluxes reaches the image pickup plane at certain timing.
An image signal (video signal) output from the image pickup element 621 is processed by a camera signal processing circuit 622 and supplied to an A/D converter circuit 624 via a signal line 623 to be converted into a digital signal in conformity with specifications such as NTSC and output to the external via a signal line 625.
This signal is displayed by a display system (not shown) compatible with a stereo image so that stereo images can be viewed.
A flow of signals for opening and closing the liquid crystal shutters will be described.
An image signal input to a sync separation circuit 701 via the signal line 625 is separated into a vertical sync signal Vsync and an odd/even judgement signal (ODD/EVEN signal).
The vertical sync signal Vsync is input to two delay circuits 702 and 703 via a signal line 705 to generate two different delay pulse signals. An output of the delay circuit 702 is used as a trigger signal for opening the shutter, and an output of the delay circuit 703 is used as a trigger signal for closing the shutter. The ODD/EVEN signal is supplied to a signal line 706.
In accordance with the two trigger signals and the ODD/EVEN signal supplied via the signal line 706, a logic circuit block 704 generates two pulse signals for driving the liquid crystal shutters 502R and 502L to open and close the shutters via a drive circuit (for right eye) 803R and a drive circuit (for left eye) 803L.
The delay circuits 702 and 703 are each constituted of a mono multi-vibrator circuit.
FIG. 11 is a diagram showing the structure of a conventional video camera simplifying the structure shown in FIG. 9. In FIG. 11, reference numeral 1001 represents right and left shutters (right eye shutter 1001R and left eye shutter 1001L) which are opened and closed at proper timings to time-divisionally receive a stereo image. Reference numeral 1002 represents right and left mirrors (right eye mirror 1002R and left eye mirror 1002L) which adjust the directions of right and left optical axes. Reference numeral 1003 represents a prism for the division of right and left optical axes.
Reference numeral 1004 represents a lens for inputting an image. Reference numeral 1005 represents a CCD for receiving a stereo image from the lens 1004. Reference numeral 1006 represents a video processing circuit which processes a signal supplied from CCD 1005 to generate a video signal Vout.
Reference numeral 1007 represents a sync separation circuit which generates a vertical sync signal (and an ODD/EVEN signal). Reference numeral 1008 represents a correction circuit which corrects drive signals by using a delay amount preset based upon an accumulation time of CCD 1005.
The corrected signals are used as drive signals of the shutters so that a stereo image without crosstalk can be input.
In the stereo image pickup system described above, the shutters for switching between right and left images are driven by right and left inverted shutter drive signals which are generated in accordance with a sync signal separated from an input image signal.
There is a difference between the sync signal and the accumulation timing of CCD. If the shutters are driven directly by drive signals not corrected, crosswalk occurs between the right and left images. From this reason, the shutter open/close timing is corrected based upon the accumulation timing of CCD.
The structure described above is, however, associated with the following problems.
The open/close timing of the liquid crystal shutter 502 shown in FIG. 9 is fixed based upon the sync signal derived from the image signal (fixed by settings of the delay circuits 702 and 703).
This relation is illustrated in FIG. 10 which is a timing chart of each signal waveform in FIG. 9. Referring to FIG. 10, reference numeral 901 represents the waveform of the vertical sync signal Vsync derived from an image signal. In accordance with this vertical sync signal, the liquid crystal shutter drive signal is generated.
Reference numeral 903 represents the waveform showing the accumulation timing of CCD which is fixed to a predetermined relation to the vertical sync signal Vsync.
Reference numeral 902 represents the waveform showing the open/close timing of the liquid crystal shutter. The sufficient conditions for the shutter open period are that this period includes the CCD accumulation timing indicated by the waveform 903, and the necessary conditions for the shutter open period are that this period does not overlap another CCD accumulation timing preceding or following the subject CCD accumulation period (right eye period for left, and left eye period for right).
If the former sufficient conditions are not satisfied, the exposure amount of an image may become insufficient, whereas if the latter necessary conditions are not satisfied, a crosswalk may occur between right and left images and the quality of a stereo image is degraded. In the conventional example, the relation between the vertical sync signal and the CCD accumulation timing is fixed. Therefore, the timing when the liquid crystal shutter is opened (after an elapse of a time Ta from the leading edge of the vertical sync signal Vsync) and the timing when the liquid crystal shutter is closed (after an elapse of a time Tb from the leading edge of the vertical sync signal Vsync) are fixed by the settings of the two delay circuits 702 and 703 shown in FIG. 9.
A lens exchange system is used on the assumption that a new lens unit and a new camera unit will be developed in the future. A newly developed camera unit does not necessarily use the same image pickup element such as a CCD, the same driver circuit and the like.
Namely, there is a possibility that the CCD accumulation time is altered.
Depending upon the specification of a high speed shutter and the like, the accumulation time may dynamically change.
In such a case, there is a mismatch (shift) between the accumulation time and the fixed open/close timing of the liquid crystal shutters so that a light amount lowers or crosstalk occurs.
Waveforms 904 and 905 shown in FIG. 10 show difference between the timings of the liquid crystal shutters and the CCD accumulataion timing when CCD accumulation times are changed. In FIG. 10, areas A indicated by solid oblique lines correspond to insufficient light amounts which result in a degraded image quality. If the difference becomes larger, the accumulation time overlaps the accumulation time for the other liquid crystal shutter, which results in crosstalk.
In correcting the difference caused by the lens unit, it is necessary to set again the correction circuit 1008 shown in FIG. 11.